1. Field of the Invention
This invention relates to a process for preparing thiourea dioxide (hereinafter referred to as "T.U.D.") and more particularly to an improved process for preparing T.U.D. in high yield and high purity in the production of T.U.D. by the reaction of thiourea and hydrogen peroxide.
2. Description of the Prior Art
The production of T.U.D. by the reaction of thiourea and hydrogen peroxide is already well known, and various methods have been proposed to improve the reaction yield and obtain products of high purity. For example, maintaining the reaction temperature in a certain range, improving the method of feeding thiourea and hydrogen peroxide, controlling the molar ratio of thiourea to hydrogen peroxide in the reaction solution, maintaining the pH of the reaction solution neutral or weak acid, etc. Thus, mainly known are methods of preventing the production of side reaction substances during reaction and suppressing the decomposition of T.U.D. However, all these methods are unsatisfactory as the method of preventing the production of by-products during reaction and suppressing the decomposition of T.U.D., and they afford only low yield and low purity of the object product, and a satisfactory result has not been obtained.
Solvents used in these methods can be broadly classified into non-aqueous and aqueous solvents.
The method of preparing T.U.D. by the reaction of thiourea and hydrogen peroxide in a non-aqueous solvent (chlorinated solvents such as carbon tetrachloride and chloroform, as well as lower aliphatic alcohols) has been proposed by German Pat. No. 917553, Italian Pat. No. 579119 and French Pat. No. 2040797. According to the said method, T.U.D. as the reaction product does not dissolve in the solvent, so that all the T.U.D. produced can be recovered as product. However, despite such an advantage, the quantity of by-products is large because the reaction of thiourea and hydrogen peroxide is a heterogeneous reaction. Besides, such by-products are incorporated into the product T.U.D. since they do not dissolve in the solvent, thus causing the purity of the product to deteriorate. Furthermore, there is the loss of solvent used. Because of these demerits, an industrial adoption of the said method is disadvantageous.
On the other hand, the production of T.U.D. in an aqueous solvent is advantageous in that since the starting thiourea dissolves well in water and forms a homogeneous phase, the quantity of by-products is small and a relatively high purity T.U.D. is obtained. Because of such a merit, this method is mainly adopted at present, and studies are being made about the improvement of this method.
U.S. Pat. No. 2,783,272 proposes maintaining the pH of the reaction solution at 2 to 6 as the method of improving the yield and purity of product. According to the said U.S. patent, at pH values below 2 of the reaction solution, T.U.D. is not produced with formation of only disulfide, while at pH values above 6, the T.U.D. produced undergoes hydrolysis, resulting in a remarkable reduction in yield. To prevent such a drawback, the said U.S. patent proposes maintaining the pH of the reaction solution at 2 to 6 by making unreacted thiourea present 0.5 to 1.0% in the reaction solution.
However, the method of the said U.S. patent cannot produce T.U.D. in high yield and in high purity. Because, the reaction of thiourea and hydrogen peroxide is so fast and the reaction heat is so high that a local generation of by-products is unavoidable; besides, by the generation of the by-products the decomposition of T.U.D. proceeds as time goes by, thus allowing production of a large quantity of sulfuric acid. Therefore, by merely changing the method of addition of thiourea and hydrogen peroxide and making unreacted thiourea present 0.5 to 1.0% in the reaction solution it is impossible to suppress a side reaction, nor is it possible to prevent the lowering of pH of the reaction solution. In addition, such a reduction in pH value causes production of a large quantity of disulfide. The disulfide thus produced forms a salt of low solubility with the sulfuric acid by-produced, which is incorporated into the product T.U.D. and causes its purity to deteriorate.
U.S. Pat. No. 3,355,486 proposes a method in which the reaction time of thiourea and hydrogen peroxide is kept within 7 minutes to minimize the by-production of sulfuric acid and to complete the reaction before production of sulfuric acid, and the pH of the reaction solution is maintained in the range of from 3 to 7. However, the yield of T.U.D. produced according to this method is extremely low because of an extreme short reaction time.
Furthermore, Japanese patent publication No. 17665/1970 proposes a method in which the reaction solution after completion of reaction and after separation of crystals is treated with a carbonate or hydroxide of an alkaline earth metal to remove sulfuric acid which hinders the reaction, and the reaction solution thus treated is reused as the next reaction solution whereby the T.U.D. dissolved in the reaction filtrate is recovered and the yield of T.U.D. improved.
The above method cannot prevent the by-production of sulfuric acid during reaction and does not remedy the drawbacks associated with the prior art process, that is, disulfide as a reaction by-product forms a salt with sulfuric acid, which is incorporated into the product T.U.D. and thereby deteriorates the purity of the product. The above method merely aims at improving the yield of T.U.D. by recovering the T.U.D. dissolved in the reaction filtrate. According to such method, however, an alkaline earth metal enters into the reaction solution, thus not only promoting the decomposition of hydrogen peroxide, but also causing the basicity of an alkali agent to become too high, so that the decomposition of T.U.D. as well as the neutralization of sulfuric acid which is the primary object is promoted, which causes lowering of the yield and purity of the product. This drawback is pointed out in Japanese patent public disclosure No. 62934/1975.
Consequently, in the conventional methods the yield of the object substance is 70% or so and the purity of product is also low, which is 96% or so, and it is difficult to further improve the yield and purity of product, and in many cases the results are unsatisfactory.
Thus, in the production of T.U.D. by the reaction of thiourea and hydrogen peroxide according to the prior art, it is apparent that if the reaction is carried out while maintaining the pH of the reaction solution in the range of from 2 to 7, a high purity T.U.D. can be prepared in high yield, and also it is easily inferable that this object can be attained if by-produced sulfuric acid, etc. are neutralized by adding a neutralizer or a weak acid pH buffer agent into the reaction solution.
In view of the above point, we have actually added the following alkaline neutralizers and weak acid pH buffer agents into the reaction solution and thereby made experiments as well as studies of a wide range: mono, di, or tri sodium, potassium or ammonium salt of a primary, secondary or tertiary phosphoric acid, sodium, potassium or ammonium salt of carbonic acid, sodium, potassium or ammonium salt of bicarbonic acid, caustic soda, caustic potash, and aqueous ammonia. However, they did not contribute to the improvement in yield and purity of the product T.U.D.
This is because in case an alkaline neutralizer is added, T.U.D. as the reaction product easily decomposes due to the presence of an alkali, and also because in the case of using a weak acid substance as a pH buffer agent, a large quantity of the pH buffer agent must be added in order to maintain the pH of the reaction solution in a preferred range, resulting in that the purity of the product T.U.D. deteriorates.
Thus, it became clear that merely by keeping an optimum pH range by the use of a neutralizer or a pH buffer agent it is difficult to obtain a high purity T.U.D. in high yield.